The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In different lighting fields, and more particularly in that of lighting shop window and shelves, it is important to provide a very high-quality lighting, which results in a high CRI index (color rendering index), that is to say at least greater than 90.
LED filaments are disclosed in United States Published Application No. US 2014/0369036 A1 filed by the Company Shenzen Runlite Technology Co., Ltd. These LED filaments are also marketed by this same Company under the reference “LED FILAMENT”, and particularly under the product codes FSS1-JG-CEEF-D0, FSS1-JG-NBFF-D0, FSS1-JG-WJDF-D0, FSS1-JG-WIDF-D0, FSQ1-JG-CEAF-D0, FSQ1-JG-WIAF-D0 and FSQ1-JG-WELJ-D0.
With reference to FIGS. 1 to 4, a LED filament 1 comprises a substrate 10 in the form of an elongate flat bar or rod, a plurality of blue 11 and red 12 light-emitting diodes 12 regularly distributed in line on the substrate 10, these diodes 11, 12 being connected successively in sequence and in series by means of electrical wires 13. The LED filament further comprises an envelope 14 overmolded around the diodes 11, 12 and the substrate 10. The two opposite ends of the substrate are provided with two metal electrodes 15, 16 forming respectively the anode and the cathode of the LED filament 1, the electrodes 15, 16 being fastened on the substrate 10, in the extension of said substrate, and being electrically connected respectively to the first and last diodes placed on the substrate 10 by means of corresponding electrical wires. The electrodes 15, 16 projecting from the envelope 14.
As shown in FIG. 2, the diodes 11, 12 may be distributed only on a lateral face of the substrate 10. Alternatively, and as shown in FIG. 4, the diodes 11, 12 may be distributed along two lines on the two opposite lateral faces of the substrate 10.
The substrate 10 is, for example, made of a material transparent to the radiation of the diodes 11, 12, to promote a 360°-lighting around the envelope 14, such as a ceramic or plastic material.
The envelope 14, placed over the entire periphery of the substrate 10, forms a protective layer which is made of a transparent colloidal gel or material containing fluorescent particles, and in particular phosphorus particles.
The overall length LT of the LED filament 1 is conventionally comprised between 10 and 150 millimeters, the substrate 10 has a width LS comprised between 0.5 and 2 millimeters and a thickness ES comprised between 0.1 and 1 millimeters, the envelope 14 has a cross-section substantially circular along a diameter DE comprised between 1 and 5 millimeters. It is known, in particular from the documents CN104406068, CN104377194 and CN204201571, to use such LED filaments inside a single-ended light bulb, as a replacement for an incandescent filament. To this end, the LED filaments are disposed side by side inside the bulb of the lightbulb, and connected respectively to the cap and to the central base of the lightbulb.
However, this use of LED filaments has several drawbacks, the first of which is that the bulbs are generally large-sized and have fixed geometrical and dimensional features, limiting their integrations in shop windows and shelves due to an excessive bulk. Furthermore, these LED filament lightbulbs are not sealed, and their sealing is often very complex or even not achievable.
It is further known from United States Patent Publication No. US 2014/375,201 to arrange, within a bulb of lightbulb, a LED filament inside a transparent heat convection tube, or even to arrange several LED filaments connected in series and surrounded by respective tubes which are brought closer at their ends. This arrangement is probably adapted for a lightbulb, but is completely unsuitable to make a miniature and adjustable linear lighting device.
United States Patent Publication No. US 2013/141,892 proposes to connect several LED filaments in series and to arrange them inside a transparent tube. However, this connection in series is unsuitable for a stable and long-lasting operation, in particular in case of failure of a diode within a LED filament.